1. Field of the Invention
The present invention relates to a process of preparing expandable styrene polymer of controlled particle size.
2. Description of the Background
In the preparation of expandable polystyrene by aqueous suspension polymerization, a relatively broad particle size spectrum is obtained. About 90% of the particles are in the range between 0.5 and 2.0 mm, with a maximum between 0.7 and 1.0 mm.
Different particle fractions have different applications. Thus, fine material is employed in the packaging sector, while coarse material is used primarily in the building sector in the manufacture of insulation, footstep insulation and the like products.
Market requirements are constantly changing. It is therefore desirable to control the particle size distribution of expandable polystyrene during polymerization without endangering suspension stability. The only possibility known to date for control is the qualitative and quantitative variation of the suspending agent composition as described in German Offenlegungsschrift 331,569, German Offenlegungsschrift 331,570, JA 945 248 and German Offenlegungsschrift 3,728,044. Influencing the particle size distribution in a controlled manner during suspension polymerization of styrene is not possible in these processes; the amount of suspending agent must be redetermined for each subsequent batch. Establishing coarse particle sizes in a controlled manner leads again and again, in the critical polymerization range of 120 to 180 min, to instabilities, which are counteracted by premature restabilization. However, this generally results in increased internal water contents and a very broad particle size spectrum with the particulate material containing a large amount of fine material.
The reason for the occurrence of instabilities and poor reproducibility are likely attributable, inter alia, to the characteristics of the particle size development during the suspension polymerization. The initially very fine particles grow only insignificantly in the first 120 minutes of polymerization time. Thereafter, a drastic growth in particle size occurs within a short time of 120 to 180 min until the desired final particle size is reached at the settling point. At the settling point, the styrene conversion is about 70%. The viscosity of the particles at this point in time is so high that virtually no coalescence occurs and formation of further particles by subdivision take place. In other words, the particles retain their identity. Immediately before the settling point, the suspension is relatively unstable and the tendency to cream increases with decreasing amount of suspending agent. Furthermore, this course of particle size development results in poor reproducibility of the batches.
An alternative is seed polymerization. A completely polymerized particle of a defined size is taken and a certain amount of organic phase is metered and disclosed in European Patent 102,655; U.S. Pat. No. 1,54,184; French Patent 2,238,717; French Patent 2,238,718; German Offenlegungsschrift 2,338,133. This process makes it possible to influence the particle size during polymerization by means of the amount of organic phase metered in. However, seed polymerization requires the presence of a seed particle free of coating agent. This preparation necessitates a separate polymerization process and hence reduces the space/time yield. Homogeneous distribution of the subsequently delivered organic phase requires a very slow metering rate. Otherwise, undesirable formation of further particles, particularly as fine material occurs. A need therefore continues to exist for a styrene suspension polymerization process which provides greater control of polymer particle size distribution. cl SUMMARY OF THE INVENTION
Accordingly, one object of the present invention is to provide a process for controlling the particle size during suspension polymerization which can be conducted in a stable manner in the critical range and by means of which a coarse final particle size can be achieved.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for preparing expandable polystyrene of controlled particle size by polymerizing styrene and optionally polymerizable comonomers in a stirred aqueous suspension in the presence of monomer-soluble free radical initiator and dispersant to the extent that at least 70%, based on the total monomer, of the monomers are polymerized in the aqueous suspension initially to a conversion of at least 70% by weight, and then adding the remaining monomer, initiator and optional copolymerizable monomer and additives to the polymerization medium over one to three hours.